This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which image nonlinear signals in the fundamental frequency band.
U.S. Pat. No. 5,879,303, of which I am a co-inventor, describes methods and apparatus for doing harmonic ultrasound imaging. As explained in my patent, an ultrasonic wave can be transmitted at a fundamental frequency to give rise to harmonic echo signals, in particular at the second harmonic, from two distinct sources. One is the nonlinear behavior of microbubble contrast agents. When these microbubble agents are insonified by the transmit wave, they will oscillate or resonate nonlinearly, returning a spectrum of echo signals including those at the second harmonic of the transmit frequency. The strong harmonic echo components uniquely distinguish echoes returning from the microbubbles, which can be used to form B mode or Doppler images of the bloodflow infused by the contrast agent. The other source of harmonic echo signals is the nonlinear distortion which ultrasonic waves undergo as they travel through tissue. The echoes returned from these distorted waves manifest harmonic components developed by this distortion.
My aforementioned U.S. patent describes two ways in which the harmonic components of these echo signals may be detected. One is by use of a highpass filter, which will pass signals in the harmonic band while attenuating the stronger echo components in the fundamental band. The other way is by transmitting two or more pulses of opposite phase or polarity and combining the echoes received in response from the two pulses. The fundamental components, being of opposite phase or polarity by reason of that characteristic of the transmit pulses, will cancel. The harmonic components of the combined echoes, being quadratic in nature, will additively combine, leaving the separated second harmonic signals.
As discussed in my aforementioned patent, harmonic signals are advantageous in many imaging situations because of the distinctive way in which they identify echoes returned from harmonic contrast agents. When used without contrast agents the tissue harmonic signals are advantageous because their development within the body eliminates much of the clutter caused by nearfield effects. However, harmonic signals are of a significantly lower amplitude than the fundamental signal echoes, providing lower signal to noise ratios and requiring greater amplification. In addition, harmonic signals require the use of relatively low frequency transmit pulses so that the second harmonic echo signal will be of a frequency which can be received within the transducer""s passband. Generally, the transmit signal will be centered at the lower end of the transducer""s passband so that the second harmonic return signal will be below the upper cutoff of the transducer passband. This can place the transmit and receive signals at the extremes where broadband signals will experience attenuating rolloff. It also mandates lower frequency transmit signals, which can be more disruptive to microbubble contrast agents than higher transmit frequencies would be. Accordingly it is desirable to be able to overcome these deficiencies and limitations of harmonic imaging.
In accordance with the principles of the present invention, the nonlinear signals returned from tissue and contrast agents are detected in the fundamental frequency band rather than at harmonic frequencies. In a preferred embodiment the nonlinear signals are detected by an amplitude modulated two (or more) pulse technique. Preferably the transmit pulse waveforms are of opposite phase and polarity and of different amplitudes. Upon reception the echoes are normalized for the different transmit amplitudes and combined and the signals within the fundamental band are used for imaging.